The present disclosure relates to a positive electrode active material containing a composite oxide including lithium and a transition metal as a constituent element, a positive electrode and a lithium ion secondary battery that use the positive electrode active material, an electric tool and an electric vehicle that use the lithium ion secondary battery, and a power storage system.
In recent years, a small-sized electronic apparatus represented by a portable terminal device or the like has become widespread, and a further reduction in size and weight, and a long operational lifespan are strongly demanded. Along with this, a development of a battery as a power source, particularly, a secondary battery, which is small in size and is light in weight, and which can obtain a high energy density, has been progressed. In recent years, this secondary battery has been reviewed for an application for use in a large-sized electronic apparatus such as a vehicle while not being limited to a small-sized electronic apparatus.
As secondary batteries, secondary batteries using various charge and discharge principles have been proposed, but among these, a lithium ion secondary battery using occlusion and emission of lithium ions has attracted attention. This is because an energy density higher than that in a lead battery, a nickel-cadmium battery, or the like, can be obtained.
The lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution, and the positive electrode and the negative electrode include a positive electrode active material and a negative electrode active material that occludes and emits lithium ions, respectively. As the positive electrode active material, a composite oxide including lithium and a transition metal as a constituent element is widely used. The positive electrode active material, which is directly related to a charge and discharge reaction, has a large effect on a battery performance, such that various studies have been undertaken with respect to the kind and a composition of the composite oxide, or the like.
Specifically, a method in which a lithium-containing compound having a lower potential and a higher capacity density than LiCoO2 is used together with LiCoO2 to prevent a capacity from decreasing due to the taking-in of lithium to a negative electrode is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 06-342673). This lithium-containing compound includes Li2NiO2 or the like, which is expressed by a general formula LixMOy (here, M represents a transition metal such as Mo, and x/y≧0.5).
In addition, a method in which a lithium copper composite oxide is used to prevent charge and discharge capacity due to charge and discharge from decreasing is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-127911). This lithium copper composite oxide includes Li2(Cu0.8Al0.1Zr0.1)O2 or the like, which is expressed by a general formula Li2(Cu1−x−yMxDy)O2 (M is an element of one kind or more selected from elements of group 13 or the like, D is an element of one kind or more selected from Ti or the like, 0<x<0.5, 0<y<0.5, and x+y<0.5).
In addition, a method in which a lithium zinc copper composite oxide is used to increase charge and discharge capacity is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-348721). This lithium zinc copper composite oxide includes Li1.98Zn0.01CuO2 or the like, which is expressed by a general formula Li2−2xZnxCuO2 (0.01≦x≦0.49).
In addition, a method in which a lithium nickel composite oxide is used to realize a large capacity is proposed (for example, refer to Japanese Unexamined Patent Application Publication Nos. 09-241027 and 09-241026). This lithium nickel composite oxide includes Li2NiO2.2 or the like, which is expressed by a general formula Li2NiO2+y (0<y<0.3), or Li2.05Ni0.95O2 or the like, which is expressed by a general formula Li2+xNi1−xO2 (0<y< 1/7).
In addition, a method in which a first lithium layered compound in which a reinsertion amount of lithium ion with respect to an initial withdrawal amount of lithium ion with 4.5 to 3 V (lithium standard) is 80% or more, and a second lithium layered compound in which this ratio is 15% or less are used to improve a battery capacity and cycle characteristics is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-009806). The first lithium layered compound includes LiCoO2 or the like, which contains at least one kind selected from among Co, and Ni. The second lithium layered compound includes Li1.29(Ni0.33Fe0.33Mn0.33)0.71O2 or the like, which includes at least one kind selected from among Fe, Mn, and Ni.
In addition to this, as a related technology, a review with respect to a negative electrode has been made. Specifically, a method in which a lithium-containing metal compound is used as a negative electrode active material to compensate for an irreversible capacity which occurs during an initial charge and discharge is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-172954). This lithium-containing metal compound can emit lithium during at least discharge.
In addition, a method in which metallic lithium is used to compensate for an irreversible capacity and thus lithium is directly applied to the negative electrode is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2008-293954).